The hard part about implementing a distance calculator based
on zip codes, vab, is not
the code; the actual code to do that is so easy,
it's almost not worth the effort to find someone else's
implementation for the borrowing. No, the hard part is
getting an up-to-date zip code database that'll map to
useful coordinates (such as latitude/longitude) that can be
used in distance calculations.

I notice that Zipdy's description says it uses a database
"created by the US Census". Does this mean
updates are only available for this database once every ten
years? The customers I've worked with who were using Zip
codes for commercial projects (e.g., on their websites
all seem to think that having all Zip codes in the database
is important enough to pay a $1000 annual subscription fee
to ensure it's updated quarterly. Certainly, though, Zipdy
is a nice resource for those working within more limited
budgets.